Image-reproducing device



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Patented Mar. 23, 1954 UNITED STATES ATENT OFFICE IMAGE-REPRODUCINGDEVICE Constantin S. Szegho, Chicago, Ill., assigner to The RaulandCorporation, a corporation of Illinois 14 Claims.

This invention relates to image-reproducing devices and moreparticularly to electrostatically focused cathode-ray tubes foi` use aspicturereproducing devices in television receivers and, the like.

The great majority of commercially produced picture tubes for televisionreceivers have been of the magnetically focused type, althoughelectrostatic focusing has been employed for special applications suchas cathode-ray oscillographs, radar, electron microscopes and the like.Magnetic focusing has been preferred for television picture tubes, inspite of its inherently higher cost, primarily for its readyadjustability, comparative freedom from lens defects such as spher- 1ical aberration, astigmatism and coma, and adaptability to massproduction techniques. On the other hand, electrostatic focusing systemsare attractive for their economy and simplicity, as

well as for the reduction in space and weight requirements and materialconservation as compared with magnetic focusing arrangements.

One of the chief deterrente to the use of electrostatic focusing systemsin television picture tubes has been the commercial adoption of iontraparrangements for removing negative ions from the electron beam toprevent fatigue of the fluorescent screen at an excessively rapid rate.While several types of ion-trap arrangements have been devised andemployed commercially, a

characteristic of each is that the mixed beam of electrons and ionsoriginating at the cathode is directed along a path displaced at leastin part from the tube axis, and the electrons alone are subsequentlydeflected to the axis by means of one or more external magnetic fields.In order to obtain quality of focusing with an electrostatic systemcomparable to that achieved by magnetic arrangements, it is necessarythat the electron beam be centrally projected in an axial directionthrough the electrostatic focusing lens; otherwise, excessiveastigmatism and/or coma are encountered. These defects are accentuatedwith an increase in the strength of the individual lens components ofthe focusing system employed.

While several distinct types of electrostatic -focusing systems areknown, the only one which has been commercially adapted for use inmodern television picture tubes is the so-called unipotentialelectrostatic lens system. Fundamentally, such a lens system comprisesthree electrodes, the outermost two of which are operated rat a commonhigh positive potential, while the intermediate one is operated at alower poten-- tial to provide the desired electrostatic eld distributioncomprising two convergent lens components and an intermediate divergentlens component having a net convergent effect on the electron beam. Ingeneral, the potential of the intermediate electrode has beenestablished at from 20% to 25% of the final anode voltage. In order toemploy a picture tu'be comprising a focusing arrangement of this type ina television receiver, it is necessary to provide special circuitcomponents in the receiver to develop the required focusing voltage.While it is generally recognize-d that an electrostatic focusing systemcapable of providing optimum focus at focusing voltages of 509 volts orless, corresponding to the ordinary direct-voltage supply or B-supply ofthe receiver, would permit the elimination of the additionalfocusing-voltage supply circuit, no commercially acceptable televisionpicture tube having an ion-trap type of electron gun and provided withsuch a focusing arrangement has been heretofore proposed. Indeed,statements may be found in the literature to the effect that an ion-trappicture tube having a focusing system which meets these specificationsis beyond the realm of commercial feasibility.

It is a primary object of the present invention to provide a new andimproved image-reproducing device of the cathode-ray type, having anion-trap type of electron gun, which employs an electrostatic focusingsystem requiring no operating potential between that of the receiver B-supply and the nal anode voltage and yet is capable of affording beamfocusing comparable to that obtainable with the best magneti-c focusingarrangements.

`The present invention is largely predicated on the principle thatundesirable lens distortion, characterized generically as multiplicityof focus and including specifically spherical aberration, astigmatism,and coma, is greatly aggravated as the strength of the individual lenscomponents of the focusing system is increased and may be effectivelyminimized, even for exceedingly strong electrostatic lens components, byaccurate centering of the electron beam. The problem is particularlyaggravated in the case of television picture tubes employing ion traps,since the electron beam must be directed to the axis from a pathdisplaced therefrom after separation of the ions. Consequently, thequality of focusing is largely dependent on the accuracy of alignment ofthe ion trap. Precise ion trap alignment by ordinary methods most widelyemployed in industry at the present time is extremely diflicult and timeconsuming, even with a highly skilled operator, and as a practicalmatter, is quite impossible of achievement on a large scale productionbasis; the best that may be achieved in commercial practice is analignment within a range of operable conditions. Slight misalignment ofthe ion trap may be tolerated in a system employing a focusing voltageof 20% or more of the nal anode voltage, but results in prohibitivelygreat lens distortion with a focusing system employing stronger lenscomponents.

In accordance with the present invention, a new and improvedelectrostatically focused image-reproducing device of the cathode-raytype comprises an evacuated envelope having av uorescent-screen-bearingtarget portion and a neck portion which is at least partly transparent.An apertured diaphragm is positioned with its aperture symmetrical withrespect to a reference axis extending longitudinally of the envelope. Anelectron gun is housed in the neck portion of the envelope and includesa cathode, a control electrode, and rst and second acceleratingelectrodes for projecting an electron beam toward thefluorescent-screen-bearing target portion of the envelope, theseelectrodes being adapted to cooperate with a predetermined magnetic eldto constitute an ion trap for intercepting negative ions originating atthe cathode and for directing electrons through the diaphragm aperturealong the longitudinal reference axis from a path displaced from thataxis. An electrostatic focusing system comprises an additional electrodeadapted to be operated at a high potential with respect to that of thecathode and a lens electrode adapted to be operated at r a potentialless than 5% of the first-mentioned potential, the additional electrodeand the lens electrode being disposed in mutually coaxial relation withrespect to the longitudinal tube axis. A fluorescent coating is includedwithin the neck portion of the envelope and is associated with theelectron gun for accurately indicating the condition of establishment ofthe predetermined magnetic field required for accurate axial alignmentof the electron beam in the diaphragm aperture to effect optimumfocusing of the electron beam on the fluorescentscreen-bearing targetportion without excessive multiplicity of focus.

As employed throughout the specification and the appended claims, theterminology a potential substantially equal to that of the cathode, asapplied1 to the opera-ting potential for the focusing lens electrode, isto be construed as meaning an operating potential which differs from thecathode potential by no more than the ordinary B-supply voltage employedin the associated apparatus. Conventional television receivers utilizeB-sunply voltages ranging from about 250 to about 500 volts and a nalanode voltage for the picture tube of from 8 to 14 kilovolts.Conseouently, it is apparent that a focusing voltage of 590 volts orless is generally less than 5% of the nal anode voltage and may properlybe considered substantially equal to the cathode potential by comparisonwith the nal anode voltage. The term low potential is employed todescribe a voltage not exceeding that of the receiver B-supply. Alow-potential" electrostatic focusing system is one which requires. nooperating voltage intermediate the B- supply voltage and the final anodevoltage.

The features ofthe present invention which are believed to be novel areset forth with particularity in the appended` claims. The inveni tion,together with further objects and advantages thereof, may best beunderstood, however, by reference to the following description taken inconnection with the accompanying drawings, in the several figures ofwhich like reference numerals indicate like elements, and in which:

Figure 1 is a fragmentary side elevation, partly in cross-section andpartly cut away, of an image-reproducing device constructed inaccordance with the present invention;

Figure 2 is an exploded perspective view of one of the electrodes of thedevice of Figure 1;

Figure 3 is a schematic diagram of a television receiver embodying theimage-reproducing device` ofV Figure 1; and

Figures 4-9 are graphical representations of operating characteristicsof the device of Figure 1.`

The image-reproducing device of Figure 1 comprises a fluorescent screenIl] afxed to the glass target portion H of a; cathode-ray tube envelopewhich also comprises a glass neck portionv |72 enclosing an electron gunand an electrostatic focusing system. The electron gun comprises acathode I3, a control electrode I4, and first and second acceleratingelectrodes I5 and I6 respectively. A diaphragm I7 having a centralaperture I8 is disposed across the outlet end of second acceleratingelectrode I6, and aperture I8 is symmetrically centered with respect tothe tube axis A-A perpendicular to the center of the uorescent screenID. Second accelerating electrode I5 is laterally offset from firstaccelerating electrode I5 to provide a transverse electrcstaticdeflection field in the region between these two electrodes, and theentire electron gun structure is tilted with respect to the. tube. axisA-A by an angle 0.

An electrostatic focusing system of the unipotential lens type is`disposed between the electron gun and the fluorescent screen. Thefocusing system comprises diaphragm Il, a lens electrode I9, and anadditional electrode 2!) which are all coaxially mounted with respect tothe tube axis A-A. Diaphragm I1 and additional electrode 2'0 aremaintained at a common operat'ng potential by means of connecting strips2|, while lens electrode I9 is provided with 'a separate lead 22extending through the base 23 of the tube, Additional electrode 20 isfurther maintained at a common potential with a conduotive coating 24 ofaquadag or the like on the inner wall of the tube envelope, by means ofetal spacer springs 25. Conductive Acoating 24 extends toward the baseonly as far as electrode 2'1 to avoid undesirable spark dischargebetween that coating and lens electrode I9, and lead 22 may be providedwith an insulating glass bead (not shown) to prevent spark discharge toelectrode I6.

For convenience, electrodes I4, I5. I6, I9 and 28 may be termed gridsand may be designated by number starting with control electrode I4 asthe first grid and progressing in the'direction of beam travel toadditional electrode 20 which is tle fth grid. All five grids aresupported in predetermined mutually spaced relation by means of a pairof glass pillars 21B, of which only one is shown, in a manner which willbe apparent to those skilled in the art. Separate leads for grids I, 2and 4 extend through the base 23 of the tube, as do the supply leads forthe cath- 0de I3 and its associated heater element (not shown).Operating voltage for the conductive coatingV 24',- and therefore forthe third-and fifth grids, may be supplied by means of a conventionalcontact button if the envelope is of the all-glass type, or directly tothe metal cone member if the tube is of the glass-metal variety.

An external permanent magnet 2l, supported in a spring clamp 28 whichfits snugly around the neck of the tube and is movable both axially androtationally, is provided to develop a magnetic eld within the tube toprovide separation of the negative ions from the electron beam.Moreover, a fluorescent coating 29 on the outer surface of the secondaccelerating electrode I9 (grid 3) is provided for facilitatingalignment of ion-trap beam-bender magnet 21.

The tube is evacuated, sealed and based in accordance with well-knownprocedures which require no further explanation, and suitable getters 30are supported from grid 5 to absorb residual gases after evacuation.

In operation, a mixed. beam of electrons and negative ions originatingat cathode I3 is projected through the aperture in first acceleratingelectrode I 5. When the mixed beam emerges from grid 2, it encounters anelectrostatic eld having a transverse component due to the lateraloffset of grid 3 with respect to grid 2. Consequently, electrons andions are both deflected upwardly in the view of Figure 1. The magneticfield imposed by beam-bender magnet 2'! serves to deflect the electronsin a downward direction as viewed in Figure 1 without substantiallyaffecting the path taken by the negative ions. rIlhus, when beam-bendermagnet 2'! is accurately adjusted, the beam of electrons is projectedcentrally through aperture I8 of diaphragm l1 in a direction along thetube axis A--A, while the negative ions are intercepted by the metallicportions of grid 3 and diaphragm I1. The ion-trap mechanism is disclosedand claimed in the copending application of Willis E. Phillips et al.,Serial No. 156,746, led April 19, 1950, now Patent No. 2,596,508, issuedMay 13, 1952, for "Electron Gun .for Cathode-Ray Tubes and assigned tothe pres- The general construction and operation of lenses of this typeare Well understood by those skilled in the art as indicated by anarticle entitled AMeasured properties of strong unipotential electronlenses by G. Liebmann, Proceedings of :the Physical Society, section B,volume 62, part 4, pages 213-228 (April 1, 1949) The required operatingvoltage for the lens .electrode (grid 4) is determined by the dimen:sions of and the spacing between the electrodes constituting theunipotential lens. `relationships are not necessarily linear, the re-Althougfh the quired focusing voltage varies directly with the lengthand inversely with the diameter of grid 4,

and inversely with the separation between grid 4 and grids 3 and 5.Certain limitations on these .parameters are imposed by practicalconsideran tions; if the diameter of grid e' is made too small,excessive spherical aberration is encountered, and if the-separationbetween grids 3 and di is made too great, the deflecting influenceexerted by the asymmetrical electrostatic field established between leadwire 22 and grid 3 becomes objectionable.

Unipotential electrostatic lens systems have lpreviouslybeen employed incathode-ray tubes.

6 Such lens systems have been found quite satisfactory and readilyadaptable to mass production techniques when the electron gun and thefocusing system are coaxial and the path of the beam is restricted tothe tube axis during its entire progress from the cathode through thefocusing system. 'However, according to present commercial practice,nearly all television picture tubes are provided with an ion-trapmechanism of one sort or another for removing negative ions from theelectron beam in order to avoid deterioration of the fluorescent screen.Such types of ion-trap arrangements as are commonly employed provide ionseparation by subjecting the mixed beam to opposed electrostatic andmagnetic elds; both electrons and ions are transversely deflected by theelectrostatic field, while the electrons only are substantiallydeflected in the opposite direction by the impressed magnetic field. Itis apparent that the practical requirement for ion trapping results in adisplacement of the electron beam from the tube axis, and it isnecessary that the beam be directed to the axis before it enters thefocusing system. The accuracy with which this is accomplished is ofimportance when magnetic focusing is employed or when a unipotentialelectrostatic lens system is employed with a focusing voltage sourcewhich may be varied over a relatively wide range, but slightmisalignment may be compensated by varying the focusing voltage.However, the ion trap alignment becomes much more critical whensatisfactory operation of the electrostatic focusing system is requiredwithin a narrow focusing voltage range between cathode potential and theB-supply voltage of the associated apparatus, owing to the increasedstrength of the individual lens components constituting the unipotentialfocusing system.

In general, if the electron beam is inaccurately centered or approachesthe focusing system in an angular manner, multiplicity of focus isencountered. The most troublesome form in which this manifests itself isthat of astigmatism and/or corna. In order to obtain focusing comparablewith that provided by magnetic focusing systems, while operating thelens electrode at or near cathode potential, it has been found essentialthat the electron beam be centrally directed along the axis in itspassage through the focusing system. In other words, the ion trap mustbe precisely adjusted for satisfactory focusing with a low-potentialelectrostatic lens system. Such precise alignment of the ion trap isfacili tated by providing a fluorescent coating 29 on the outer wall ofsecond accelerating electrode I6. This fluorescent coating serves as aniontrap adjustment indicator; in practice, perma nent magnet 2'! ismoved both axially and rota tionally until the glow from fluorescentcoating 2d is reduced to a minimum, thus indicating precise ion trapalignment. The fluorescent coating must of course be so situated that itis excited into fluorescence whenever the electron beam is entirely orpartially intercepted by diaphragm Il', and the fluorescent glow must bevisible, either directly or by reflection, through the transparent neckportion i2 of the tube envelope. Other possible locations forfluorescent coating 29 include the inner wall of neck portion i2 nearthe space between grids 2 and 3, the inner surface of diaphragm I1, andthe surface of diaphragm 20 facing the cathode. The ion-trap indicatoris described and claimed in .the copending applications of ConstantinlS.

Szegho, Serial No. 134,725'led December 23, 1949, now U. S. Patent No.2,564,737, issued August 2l, 1951, and of Constantin S. Szegho et al.,Serial No. 162,906, led May 19, 1950, now U. S. Patent No. 2,565,533,issuedAugust 28, 1951, both entitled Cathode-Ray Tube, and both assignedto: the present assignee.

In order to obtain satisfactory focusing with the system shown in Figure1, it is necessary to maintain rather stringent manufacturing toleranceswith respect to the dimensions and spacings of the several electrodesconstituting the focusing system. In addition, since grid 4 is to beoperated at a potential substantially equal to that of the cathode,extremely high voltage gradients are produced between grid 4 and grids 3and 5. In order to avoid undesirable corona effects and field emission,grids 3 and 5 are each provided with corona rings 3I and 32 in the formof rolled flanges of stainless steel or the like which are welded orotherwise secured to the respective electrodes, and grid 4 isconstructed by rolling the two ends of a metal cylinder 33 over the edgeof a large aperture in a metal disc 34. The particular construction ofgrid 4 is specifically claimed in the copending application of Robert W.Shawfrank, Serial No. 234,920, filed July 3, 1951, now U. S. Patent No.2,627,049, issued January 27, 1953, for Cathode-Ray Tube Electrodes andassigned to the p-resent assignee. The construction of grid 5 isillustrated in the exploded perspective view of Figure 2, and the outletend of grid 3 is similarly constructed. Grid 5 comprises a cylindricalportion 35, an outwardly extending flange portion 36 at one end ofcylindrical portion 35, and a centrally apervtured diaphragm 3'I acrossthe other end of cylindrical portion 35. Corona ring 32 is supportedinternally of cylindrical portion 35 and is provided with a rolled rim33 overlapping and terminally abutting flange portion 35. Thus, theelect-rode comprises a cylindrical portion, an apertured diaphragmacross one end of the cylindrical portion, and a substantially planarange portion extending outwardly from the cylindrical portion in a planeincluding the other end of the cylindrical portion; the electrodefurther comprises a smoothly curved corona-'inhibiting portion betweenthe cylindrical portion and the flange portion and tangential to a planeparallel 'to and spaced from the above-mentioned plane.

glass pillars 26, the gun assembly being properly oriented in the tubeneck by means of other jigs in the usual manner. It has been found thatthese precautions suiiice to insure satisfactory operation of thecompleted structure, any small deviations in dimensions and spacingsbeing readily compensated by adjustment of the ion- As compared with thesomewhat similar constructions now employed in magnetically focusedtelevision picture tubes, the electron gun of the tube of Figure l hasbeen modified in two further structural respects. In the rst place, theangle 0 by which the entire gun is tilted with respect to the tube axisA-A is reduced from about 6 de- 8; grees to about`4% degrees, and theYamount "of lateral offset of grid 3 with respect to' grid` 2 is reducedfrom 1.9 mm. to 1.6 mm. In the Vsecond place, the length of the tubularportion of grid 3 is reduced from about 38 millimeters to about 30millimeters.

These two parameters influence the performance of the electrostaticfocusing system in the following manner. In order to obtain satisfactoryfocusing with the reduced focusingvoltage range, it is desirable to makethe electron beam more nearly parallel as it enters the focusing system.This may be accomplished by locating the unipotential lens system nearerto the cathode and thereby decreasing its focal length; for this reasongrid 3 is shortened. However, it is not possible to shorten grid 3indefinitely because a certain minimum length is required to insure iontrapping at normal operating voltages. To compensate for the impairedperformance of the ion trap occasioned by shortening grid 3, the amountof lateral offset between grids 2 and 3 and the angle 0 by which theentire electron gun is tilted with respect to the tube axis A-A may eachbe increased; however, increasing the angle of gun tilt results in anincrease in the angle at which the beam enters the focusing system,resulting in greater astigmatism and/or coma, so that from this point ofView the angle of gun tilt should be made as small as possibleconsistent with the requirement for ion-trapping with a singlebeam-bender magnet. The condition represented by the relationships setforth in the preceding paragraph has been found to provide good focusingwhile retaining ion trapping at reasonable operating voltages. In thisconnection, it is observed that the particular type of offset ion trapemployed in the tube of Figure 1 has the important advantage over othertypes of ion-trap gun, such as that employing coaxial electrodes with aslanted aperture between grids 2 and 3, that a greater angle of gun tiltmay be employed for a given amount of lens distortion Ysince the cathodeis closer to the tube axis.

Moreover, the offset type of ion trap is free of the characteristicelliptical distortion associated with the coaxial slanted-aperture type.

For best results, it has been found that the apertures in grids I, 2, 3and 5 should be in marginally overlapping alignment in a directionparallel to the tube axis A-A. In other words, all of these aperturesshould intercept an imaginary straight line parallel to referenceaxisA-A, and the apertures in grids I and 2 should intercept that lineasymmetrically. Fulfillment of this condition is dependent upon theangle 0 by which lthe entire electron gun is tilted with respect to thetube axis, and also upon the length of the electron gun from the cathodeto aperture I8 in diaphragm II. If the angle 0 and/or the length of thegun is increased to such an extent that the apertures in grids I, 2, 3and 5 are no longer in marginally overlapping alignment in a direc'-tion parallel to the tube axis, increased multiplicity of focus isencountered, and the performance of the focusing system is inferior. Onthe other hand, if the angle 0 is decreased so that the apertures are incomplete coaxial alignment, ion trapping may no longer be convenientlyaccomplished with a single beam-bender magnet.

In a television receiver schematically illustrated in Figure 3, incomingcomposite television signals are received and separated into videosignalcomponents and synchronizing-signal components by means of conventional,receiver ynarily between 250 and 450 volts. range, the spot sizedecreases with increasing circuits 4U which may include aradio-frequency amplifier, an oscillator-converter, anintermediate-frequency amplifier, a video detector, a video amplifierand a synchronizing-signal separator, as well as suitable circuits forreproducing the sound portion of the received signal. The detectedcomposite video signal from receiver circuits Il@ is applied between thecontrol electrode i4 and the grounded cathode i3 of an image-reproducingdevice 4l of the type shown in YFigure 1.

synchronizing-signal components of the detected composite video signalare employed to drive a synchronizing system t2 of convention-- alconstruction which supplies the line-frequency and field-frequencydeflection coils 43 and 44 with suitable scanning currents to controlthe scansions of the cathode-ray beam of device A high-voltage powersupply 45, which may also 4be of conventional construction, is employedto provide a suitable high operating voltage for the conductive coating215 to which grids 3 and e are rinternally connected. Lens electrode i9(grid 4) is connected to a variable tap 46 associated with apotentiometer resistor 4l connected between the receiver D. C. voltagesupply source, conventionally designated B+, and ground. With the tubeconstruction shown and described in connection with Figure l, optimumfocus of the cathode-ray beam is achieved when the minimum glow ofconstant. The solid curve 5t illustrates the beam diameter versus beamcurrent characteristic of the electrostatically focused tube shown inFigure 1, while the dotted curve 5l, plotted on the same axes forpurpose of comparison, represents a similar characteristic for aconventional magnetically focused cathode-ray tube. It is apparent froma consideration of the two curves that the beam diameter is lessdependent on beam current in the case of the electrostatically focusedtube of the present invention (in other words, the beam is maintainedmore nearly parallel) than with the conventional magnetically focusedtube; While the spot size at the fluorescent screen is somewhat largerthan that obtainable by magnetic focusing, it is also more unij form.

Figure 5 is a curve of spot size at the fluorescent screen as a functionof the voltage applied to the first accelerating electrode l5 (grid 2)the remaining operating conditions being maintained constant. Grid 2 isconventionally operated at the receiver B-supply voltage, which isordivVithin this grid-2 voltage in a manner comparable with the'performance of conventional magnetically focused tubes.

Figure 5 is a plot of the focusing voltage applied to the lens electrodeI9 (grid 4) for optimum focusing conditions as a function of theoperating voltage of grid-2. It is apparent from this characteristicthat the B-supply voltage of the associated television receiver, withinthe `range ordinarily employed, has no effect on the 10 Voltage requiredat the lens electrode for minimum lens distortion.

Figure 7 is a graphical representation of the spot size at thefluorescent screen as a function of the voi-tage applied to grids 3 and5 and conductive coating i4, the beam current being maintained constantat 200 microarnperes and the focusing voltage applied to grid d beingvaried within the range of from 150 to 25() volts to maintain optimumfocusing. Figure S is a curve i1- lustrating the voltage required forgrid i to maintain optimum focusing, plotted as a function of finalanode voltage, the beam current also being maintained constant at 200microamperes. From a consideration of the curves of Figures 7 and 8, itis apparent that the tube of Figure l may be operated within wide rangeof final anode voltages while still retaining satisfactory focusing witha focusing voltage between the Al3- supply voltage and that of thepicture tube cathode.

Figure 9 is a graphical representation of the spot size at thefluorescent screen as a function of focusing voltage and beam current,the nal anode voltage being maintained constant at 14 kilovolts and thegrid-2 voltage at 300 volts. From this family of curves, it is apparentthat optimum focusing is obtained for any beam current at a focusingvoltage below 300 volts, and that the effect of beam current on spotsize for any given focusing condition is no more pronounced than in thecase of conventional magnetically focused tubes.

Merely by way of illustration and in no sense by way of limitation, itmay be desirable to tabulate certain critical dimensional relationshipsin an operative embodiment of the invention constructed in the mannershown and described in connection with Figure 1. The operatingcharacteristics of Figures 4-9 were obtained with a tube of the typeshown in Figure 1 having the following dimensional relationships:

Angle 0 of gun tilt 445' Length of grid 3 mm 30 Lateral offset of grid 3with respect to grid 2 inch .063 Longitudinal spacing between grid 2 andgrid 3 inch .080 Diameter of aperture in grid l do. .040 Diameter ofaperture in grid 2 do .075 Diameter of aperture i8 do .100 Diameter ofaperture in grid 5 do .100 Inner diameter of grid 4 do .500

Inner diameter of corona rings 3i and 32 inch .470 Axial length of grid4 do .125 Axial spacing between grid 3 and grid 4 inch .110 Axialspacing between grid 4 and grid 5 inch .110

'The dimensions of grids 3, 4 and 5 and the spac- '.000 inch; whilethese tolerances are extremely stringent, and would be practicallyimpossible to maintain with ordinary parts, the reinforcing veifect ofthe corona rings on grids 3 and 5 permit ordinary manufacturingtechniques to be em- 11 ployed while maintaining the specified closetolerances.

While the invention has been shown and described in connection with anembodimentJ which utilizes an ion-trap type electron gun comprisinglaterally offset first and second accelerating electrodes, and While theentire electron gun has been shown in a position tilted with respect tothe longitudinal axisof the' tube, the invention is also applicable tocathode-ray tubes employing other types of ion-trap arrangements. Forexample, the ion trap may be of the type comprising a pair of adjacentelectrodes having complementary slanted apertures, or of the type inwhich the entire electron gun is curved or bent, or of any other knownconstruction. Moreover, the electron gun may be mounted either on orparallel to the longitudinal axis of the tube, in which case a pair ofbeam bender magnets may be required. In any tube employing an ion-traptype electron gun, the same problems are presented vvhen it is attemptedto employ a lowpotential electrostatic focusing system; in order toobtain efcient ion-trap action, the electrons must be directed to thetube axis from a path displaced from that axis, and accurate centeringof the beam in the electrostatic focusing system is essentialifunduemultiplicity of focus is to be avoided; By employing the ion-trapindicator,

accurate axial alignment of the electron beam in f the focusing systemis insured, and satisfactory focusing may be achieved event/ith anoperating potential for the lensV electrode which is substantially equalto, that ofthe cathode.

While a particular embodiment of the present invention has been shownand described, it is apparent that various changes and modifications maybe made, and it is therefore contemplated in the appended claims tocover all such changes and modifications. as fall within the true spiritand scope of the invention.V

I claim:

1. An electrostatically focused image-reproducing device of thecathode-ray type comprising: an evacuated envelopeenclosing a targetportion and having a neck portion which is at least partiallytransparent; a diaphragm having an aperture symmetrical with respect toa reference axis extending longitudinally of said envelope; an electrongun housed in said neck portion of said envelope for projecting anelectron beam toward said target portion of saidenvelope and including acathode, a control electrode, and first and secondV acceleratingelectrodes adapted to cooperate with a predetermined magnetic field toconstitute an ion trap for intercepting negative ions originating atsaid cathodeand for directing electrons through said aperture along saidreference axis from a path displaced from said reference axis; anelectrostatic focusing system including an additionalelectrode adaptedto beoperated at a high potential with respect to that of said cathodeVanda lens electrode adapted to beoperated at a potential less than 5% ofsaid first-mentioned potential, saidradditiona-l electrode and saidlensv electrode being disposed in mutually coaxial relation with respectto Said reference axis; andra fluorescent coating within said neckportion of said envelope and associated with said electron gun foraccurately indicating the condition of establishment of saidpredetermined magnetic eld to eiect optimum focusing of said beam onsaid'target portion without excessive multiplicity of focus.

2. An electrostatically focused image-reproducing device ofthecathode-ray type comprising: an evacuated envelopeenclosing a targetportion and having a neck portion which is at least partiallytransparent; a diaphragm having an aperture symmetrical with respect toa reference axis perpendicular tothe center of said target portion ofsaid envelope; an electron gun housed in said neck portion of saidenvelope for projecting an electron beam toward said target portion ofsaid envelope and including a cathode, a control electrode, and rst andsecond accelerating electrodes adapted to cooperate-With a predeterminedmagnetic field to constitute an ion trap for intercepting negative ionsoriginatingl at said cathodeand for directing electrons through saidaperture along said reference axis from a path displaced' from Isaidreference axis; an electrostatic focusing system including an additionalelectroden adapted to` be operated at a high potential With respect tothat of said cathode and a lens electrode adapted to be operated at apotentialless than 5% of said firstmentioned potential, said additionalelectrode and said lens electrode being disposed in mutually coaxialrelation With respect to said reference axis; and aA fluorescent coatingwithin said neck portion of said envelope and associated with saidelectron gunfor accurately indicating the condition of establishment ofVsaid predetermined magnetic eld to-.effect optimum focusing of said beamon said target portion without cxcessive multiplicity of focus.

3. An electrostatically focused image-reproducing device of thecathode-ray type comprising: an evacuated envelope enclosing a targetportion and havinga neck portion which is at least partiallytransparent; a diagram having an aperture symmetricalwith respect to areference axis extending longitudinallyV of said envelope; an electrongun housed in said neck portion of said envelope for projecting anelectron beam toward said target portion of said envelope and includinga cathode, a control electrode and first and second acceleratingelectrodes adapted to cooperate vvithaV predetermined magnetic field toconstitute an iontrapfor intercepting negative ions originating at saidcathode and for directing electrons through said aperture along saidreference. axis from a path displacedfrom said reference axis; a;unipotential electrostatic focusing system comprising said diaphragm, anadditional electrode adaptedto be operated at a high potentialwith-respect to that of said cathode, and a lens electrode adapted to beoperated atA a. potential less. than 5% of said first-mentionedpotential; said diaphragm, said additional electrode, and saidlenselectrode being disposed inmutually coaxial relation with respect tosaid axis;iandra fluorescentrcoating within said neck portion of saidenvelope and associated with said electron gunfor accuratelyv indicatingthe condition` of establishment ofv said predetermined magnetic field toeffect; optimum focusing of said beam on said target portion withoutexcessive multiplicity. of focus.

4. An electrostatically focused image-reproducing device of thecathode-ray type comprising: an evacuated envelopeenclosing a targetportion and having aneck portion4 which is at least partiallytransparent; a diaphragm having an aperture symmetrical with respect toa reference axis extending longitudinally of said envelope; an electrongun housed in said neck portion of saidenvelope for projecting anelectron beam towardsaid targetportion of said envelope 13 'andincluding a cathode, a control electrode, and vfirst and secondaccelerating electrodes individually supported with their axes inclinedto said reference axis, said electrodes being adapted to cooperate witha predetermined magnetic field to constitute anion trap for interceptingnegative ions originating at said cathode and for directing electronsthrough said aperture along said reference axis from a path displacedfrom said reference axis; an electrostatic focusing system including anadditional electrode adapted to be operated at a high potential withrespect to that of said cathode and a lens electrode adapted to beoperated at a potential less than of said firstmentioned potential, saidadditional electrode and said lens electrode being disposed in mutuallycoaxial relation with respect to said reference axis; and a iiuorescentcoating within said neck portion of said envelope and associated withsaid electron gun for accurately indicating the condition ofestablishment of said predetermined -magnetic field to effect optimumfocusing of said 'beam on said target portion without excessivemultiplicity of focus.

5. An electrostatically focused image-reproducing device of thecathode-ray type comprising: an evacuated envelope enclosing a targetportion and having a neck portion which is at least partiallytransparent; a diaphragm having an aperture symmetrical with respect toa reference axis extending longitudinally of said envelope; an electrongun housed in said neck portion of said envelope for projecting anelectron beam toward said target portion of said envelope and includinga cathode, a control electrode and `a first accelerating electrode eachsupported in substantially coaxial alignment with said cathode, and asecond accelerating electrode supported with its axis parallel to thatof said iirst accelerating electrode but offset therefrom by a distancesmall in comparison with the diameters of said electrodes, saidelectrodes being adapted to cooperate with a predetermined magnetic heldto constitute an ion trap for intercepting negative ions originating atsaid cathode and for directing electrons through said aperture along`said reference axis from a path displaced from .disposed in mutuallycoaxial relation with respect to said reference axis; and a fluorescentcoating within said neck portion of said envelope and associated withsaid electron gun for accurately indicating the condition ofestablishment of said predetermined magnetic field to veffect optimumfocusing of said beam on said target portion Without excessivemultiplicity of focus.

6. An electrostatically focused image-reproducing device of thecathode-ray type comprising: an evacuated envelope enclosing a targetportion and having a neck portion which is at jleast partiallytransparent; a diaphragm having an aperture symmetrical with respect toa reference axis extending longitudinally of said envelope; an electrongun housed in said neck portion of said envelope for projecting anelectron beam toward said target portion of said v,envelope andincluding a cathode, a control electrode and a. first acceleratingelectrode supported in substantially coaxial alignment with said cathodeand with their axes inclined to said reference axis, and a secondaccelerating electrode supported with its axis parallel to that of saidfirst accelerating electrode but offset therefrom by a distance small incomparison vvithr the diameters of said electrodes, said electrodesbeing adapted to cooperate with a predetermined magnetic eld toconstitute an ion trap for intercepting negative ions originating atsaid cathode and for directing electrons through said aperture alongsaid reference axis from a path displaced from said reference axis; anelectrostatic focusing system including an additional electrode adaptedto be operated at a high potential with respect to that of said cathodeand a lens electrode adapted to be operated at a potential less than 5%of said first-mentioned potential, said additional electrode and saidlens electrode being disposed in mutually coaxial relation with respectto said reference axis; and a fluorescent coating Within said neckportion of said envelope and associated with said electron gun foraccurately indicating the condition of establishment of saidpredetermined magnetic field to effect optimum focusing of said beam onsaid target portion Without excessive multiplicity of focus.

7. An electrostatically focused image-reproducing device of thecathode-ray type comprising: an evacuated envelope enclosing a targetportion and having a neck portion which is at least partiallytransparent; a diaphragm having an aperture symmetrical with respect toa reference axis extending longitudinally of said envelope; an electrongun housed in said neck portion of said envelope for projecting anelectron beam toward said target portion of said envelope and includinga cathode, a control electrode, and first and second acceleratingelectrodes adapted to cooperate with a predetermined magnetic eld toconstitute an ion trap for intercepting negative ions originating atsaid cathode and for directing electrons through said aperture alongsaid reference axis from a path displaced from said reference axis; anelectrostatic focusing system including a lens electrode and anadditional electrode disposed in mutually coaxial relation with respectto said reference axis; means for operating said additional electrode ata high potential with respect to that of said cathode; means foroperating said lens electrode at a potential less than 5% of saidfirstrnentioned potential; and a fluorescent coating Within said neckportion of said envelope and associated with said electron gun foraccurately indicating the condition of establishment of saidpredetermined magnetic field to effect optimum focusing of said beam onsaid target portion Without excessive multiplicity of focus.

8. An electrostatically focused image-reproducing device of thecathode-ray type comprising: an evacuated envelope enclosing a targetportion and having a neck portion which is at least partiallytransparent; a diaphragm having an aperture symmetrical with respect toa reference axis extending longitudinally of said envelope; an electrongun housed in said neck portion of said envelope for projecting anelectron beam toward said target portion of said envelope and includinga cathode, a control electrode, and first and second acceleratingelectrodes adapted to cooperate with a predetermined magnetic field toconstitute an ion trap for intercepting negative ions originating atsaid cathode and for directing electrons through saidaperture-.alongsaidi. reference axis from-.a` path displaced. from;said. reference axis; a unipotential. electrostatic. focusing systemincluding, said diaphragm, a. lens electrode,nand an additionalelectrode disposed in. mutuallyY coaxial relation associated with saidelectron gun for accurately indicatingthe condition of establishment ofsaid predetermined magnetic field. to effect optimum focusing of saidbeam on said target portion without excessive multiplicity of focus.-

9. An electrostatioallyI focused image-reproduoing device ofthecathode-ray typecomprising: an evacuated. envelopeV enclosing a targetportionand having a neck` portion which is at leastv partiallytransparent; a; diaphragm having an aperture symmetrical with respect toa reference axis extending longitudinally of said. envelope; anelectronl gun housed in said neck portion .of said envelope forprojecting aneectron beam toward said target portion of said envelopeand including a cathode, a control electrode, and rstland secondaccelerating` electrodes; adjustable means for applying a magnetic fieldwhich cooperates with said'electrodes to'constitute an ion trap forintercepting negative ionsoriginating at said` cathode and. fordirecting electrons through said aperture-along said reference axis froma path displaced .from said reference axis; anY electrostatic focusing.system including an additional electrode adapted to be operated at ahigh potential with respect4 to that of said cathode and' a lenselectrode adapted to be operated at a potential less than 5% of saidiirstmentioned potential, said additional electrode and said lenselectrode being disposed in mutually coaxial relation with respect tosaid reference axis; and-aiiuorescent coating within said neel; portionof said envelope and associated with said electron gun for accuratelyindicating the reference axis extending longitudinally of said envelope;an electron gun housed. in saidneck portion ofsaidenvelope forprojectinganelec.- tron beam toward said target portion of said envelope andincluding a cathode, a controlelectrode, a rst accelerating electrode,and a second accelerating electrode including said diaphragm, saidelectrodes being adapted to cooperate with a predetermined magneticfield to constitute an ion trap for intercepting negative ionsoriginating at said cathode andl fordirecting electrons through saidaperture along said reference axis from a path displaced from saidreference axis; an electrostatic focusing system including an additionalelectrode adapted to be operated at a high potential withA respectto-that of. said cathode and a lens electrodev adapted to be operated atapotential less than 5% of said rstmentioned potential, said.additional.A electrode and saidlens electrode being disposed inLmutually coaxial relation with respect toA said-refer;- ence axis; and afluorescent coating within'said neck portion of said envelope andassociated-with said electron gunfor accurately indicatingthe conditionofestablishment of said ypredetermined magnetic eld to eect optimumfocusing of said beam on said target portion without excessivemultiplicity of focus 11. An electrodek for a cathode-ray tubecomprising: a cylindrical portion; an apertured diaphragm across one endof said cylindrical portion; a substantially planar ange portionextending outwardly from said cylindrical portion in a plane includingthe other end ofl said cylindrical portion; and a smoothly curvedcorona-inhibiting portion between saidy otherend of said cylindricalportion and said flange portion and tangential to a plane parallel toand spaced from said Erst-mentioned plane.

12. An electrode for a cathode-ray tube comprising: a cylindricalportion; an outwardlyV extending flange portion at one end of saidcylindrical portion; an apertured diaphragm across the other end of saidcylindrical portion;A and a corona ring supported internally ofsaidcylindrical portion and having a rimoverlappingand terminallyabutting said flange portion for inhibiting corona discharge betweensaid electrode and an adjacent electrode of said tube and formechanically reinforcing said electrode against warping and bending,

13. An electrode for a cathode-ray tube comprising: a cylindricalportion; a substantially planar outwardly extending ange portion atl oneendof said cylindricalportion; an aperturedcdiaphragm across the other..endiof. said.. cylindrical portion; and a corona'iring supportedinternally of said'cylindrical portionandihaving arolled rim overlappingand terminally. abuttingE said flange portion for inhibitingV coronadischarge between said electrode and an adjacent electrode of said tube'and for mechanically-'reinforcing said electrode against warping andbending.

14. An electrostatically focused image-repro.. duoing device ofthe'cathode-ray tub'e type comprising: anevacuated envelope enclosing atarget portion and having a neck portionwhich isat least partiallytransparent; a diaphragmohaving an aperture symmetrical with respect`to. a reference axis extending longitudinally of saidenve.- lope; anelectron gunjhoused insaid neck portion of said envelope-for projectingan electron beam towardsaid target portion: of. said envelopefandincluding a4 cathode; and a. pair of electrodes adapted to cooperate:with Ya'predeterminedmagnetic eld'tdconstitute anion trap forintercepting negative ions originatingat said cathode and for directingelectrons through lsaid `aperture along sa'idueferencel axis-from apath' displaced from said reference axis; an'electrostatic -focusingsystem including an additional electrode adapted'to be operatedfat ahigh potential with respectto that of saidcathode anda lens electrode.adapted. to f be .operated f at a. potential 'less than 5%, of saidfirst-mentioned potential, said additional electrode and said .lenselectrodebeing disposedinmutually coaxial relation Withrespect tosaidreference axis ;..and.a uorescent. coating Within said neck,portionl of. said. envelope.. and

associated with said electron-gunfor. accurately indicating thecondition, ofestablishment of f said predetermined magnetic heldv to .Veffect` optimum 17 18 focusing of said beam on said target portionNumber Name Date Without excessive multiplicity of focus. 2,484,721 MossOct. 11, 1949 CONSTANTIN S. SZEGHO. 2,555,850 Glyptis June 5, 19512,564,737 Szegho et al. Aug. 21, 1951 References Cited in the le of thispatent 5 2,555,533 Szegho et a1 Aug. 23, 1951 UNITED STATES PATENTS2,575,835 Pohle Nov. 20, 1951 Number Name Date 2,604,599 Breeden July22, 1952 .2,058,194 Rudenberg Oct. 27, 1936 OTHER REFERENCES 2,070,319Rudenbe'g Feb- 9: 1937 10 Article by Bowie, Proceedings of the Institutegom leb- 2g of Radio Engineers, vol. 36, No. 12, December 1948, i OgOWSugpp. 1482-1486. f 2,363,359 Ramo NOV- 21 1944 Industrial Electronicsand Control by R. G. 2,452,919 Gabor NOV- 2, 194'8 Kipefer, copyright1949, published by John W.

vudbg ITTOV- 2 15 Wiley & C0., see in particular page 453, Fig. 2. oo r1ge une

